A heat sealer is a machine used to seal products and packaging by joining materials using heat. The basic concept is to join two thermoplastic components by applying heat and pressure until the materials melt and flow together, then allowing the components to cool.
There are several different types of heat sealing devices. One particular class of devices, called impulse heat sealers, operates by holding the materials to be joined between jaws that contain one or more resistive heating elements. The materials are held in place by pressure, and heat is applied to the materials by applying an electric current to the heating elements until the materials reach a desired temperature. The heat is maintained until enough time has passed for the materials to flow together adequately. The materials are then allowed to cool, sometimes with the aid of a cooling mechanism, which allows the materials to fuse together.
The strength and quality of the seal that is formed is largely a result of using properly designed components with appropriate materials, and applying correct amounts of temperature and pressure for an appropriate amount of time.
A typical heat sealing application involves fusing two thermoplastic sheets or films to create a bag. In the simplest bags, the sheets are uniform monolayers, and a heat sealer is used to create fused edge seals.
In an application where flat films are fused, the resistive heating elements used in the heat sealer are usually straight pieces of drawn wire or ribbon that are made from a suitable material such as nichrome. The uniform cross-sectional area of such heating elements results in a uniform temperature along the element. This is due to the uniform current density that arises in the uniform resistive element at steady state when current is passed through the element. Such elements function well in simple applications because the materials to be sealed are of a uniform material and thickness.
It is sometimes desirable to seal more complex arrangements of materials. One such application is to seal an object or objects between two sheets such that the sheets form a bag, and such that the object or objects protrude out of the bag through the seal. In these more complex applications, the sheets may be sealed together in places, and sealed to the objects in other places. An example of such a complex application is shown in FIG. 1, where tubes are sealed between two poly sheets to form a medical IV bag with two ports. However this added complexity of construction has a number of implications for creating the seal.
One problem that arises is that unlike the simple case of sealing two flat films, the materials to be sealed are not uniform in mass, thickness, and/or melting or heat-sinking properties. This means that the heating requirements to form a proper seal are different along the length of the seal. For example, in FIG. 1, the portions of the seal which fuse only the two films will require less heat energy to form the seal than the portions which fuse the tubes between the films. This is because the mass of the tubes requires more heat to melt, and also acts as a heat sink, drawing heat away from the films.
Because of these effects, a standard heating element having a uniform cross-sectional area may yield undesirable results when sealing complex arrangements of components. This is because in order to use a standard heating element to form the seal, either the temperature, the time the heat is applied, or both must be increased in order to ensure that a seal is formed in the areas of increased mass.
However, none of these solutions is desirable from a production standpoint. By increasing the heating time, production is slowed and energy costs increase. Further, increased heating time alone may not be adequate in certain circumstances where higher mass parts dissipate heat into the environment rapidly. By increasing the temperature of the element, production speed may be maintained, but the temperature required to seal areas of greater mass may be too great for thinner thermoplastic components. This can result in deformation of the finished product, unreliable seals, degradation of materials, and so forth. The excess heat applied to the thinner regions in order to adequately heat the thicker regions also represents increased energy that is wasted.
Another problem in such applications arises due to the complex shape of the seal. In order to properly apply heat to the surfaces to be joined, the heating element must be shaped to follow the contours of the desired seal, which includes the parts that protrude through the seal. Typically, this is done by simply bending the wire or ribbon of resistive material into the proper shape. However, bending the element in this way can yield inadequate results.
One issue arises if the bending process is not adequate to create a sharp transition between sections of the element having different shapes. This may prevent the heating element from making proper contact with the components to be sealed in the “corners” between profile shapes, thus negatively affecting the bond.
Another issue arises if the bending process at a sharp corner strains the material locally, resulting in a thinning or distortion of the heating element at that point. This may result in an increase in temperature at the corners between shapes due to locally increased current density, which may negatively affect the bond or damage the materials.
Many other complex arrangements of materials are possible, all of which implicate the issues identified above. For example, it may be desirable to seal an article having multiple layers, having gussets in discrete areas, having areas of dissimilar materials, or having fitments or tabs (such as a hang tag) sealed into the article. It may also be desirable to apply a “cut/seal”, where an area of the sealer cuts through the materials using added heat and/or pressure.
What is desired, therefore, is a single resistive heat sealing element which possesses the true shape desired, and which also allows for a targeted temperature profile to be created therealong (i.e., higher temperatures in areas of film-to-tube sealing and lower temperatures in areas of film-to-film sealing).